Giant Ocean Vortex Linked to Monsoon

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The edge of the Great Whirl, shown by high chlorophyll concentrations along its flank.
IOCCG

No predictable pattern

The researchers analyzed the Great Whirl's habits by combining 18 years of satellite records with data collected from a research cruise in 1995. There have been very few research cruises through the Arabian Sea since 1995 because of Somali piracy, Beal said. (The World's Biggest Oceans and Seas)

The vortex lasts for roughly 166 days each year, but the team found no predictable pattern to its location and orientation.

Over the years, the Great Whirl's wanderings were caused by its own mini-cyclones, the researchers discovered. As the current spins, it creates two to three flanking cyclones along its edge. (Cyclones rotate counter-clockwise, opposite to the Great Whirl.) The interaction between the smaller cyclones and the "mother current" makes the Great Whirl move and shift around in response.

"The Great Whirls spins up these flanking cyclones because it has such high velocity shear along its edge. The water is basically rotating these cyclones clockwise around its flank, and it's causing a kind of turbulence. It's like a mutual eddy advection," Beal said.

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Flip-flopping currents

The Great Whirl is not the only strange phenomenon in the Arabian Sea. The basin is the only place in the world where the ocean's currents reverse direction every year.

"The entire circulation of the basin switches direction from summer to winter, which is really crazy. It doesn't happen anywhere else in the world's oceans," Beal said. (Video: Animation Reveals Ocean Currents)

Understanding how the region's currents respond to the monsoon winds is important because the circulation is directly linked to sea surface temperature, Beal said. As with the Pacific Ocean's El Niño, sea surface temperature is the No. 1 effect on rainfall, she said.

Because the Great Whirl brings up warm water in its core, but cold water in its smaller flanking cyclones, the current has a complex effect on climate and moisture content in the monsoon winds.

Beal plans to further explore the link between the Great Whirl and Rossby waves. "If there is some feedback between the previous monsoon and how the Great Whirl is initiated, that could give us some predictability on what the strength of the monsoon will be, and also some predictability about the rainfall, which will be important to people who live in southern Asia."

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